Need help with Total Line for Stocks

Hello,

I am a Beginner in Power BI and want to create an Overview of monthly stocks. So I have createad a Matrix, where I have the "Stock Begin", "Gross Production", "Sales", "Stock End" per month (each month is a line). 

Now, I also want to show in this Matrix the YTD result but do not have a glue, how to do it: If I use the Total Sum from Power BI, the Starting, Ending and Average Stock is wrong, because it is the Sum of each Starting Stock per month, but:

YTD Starting Stock = January Starting Stock

YTD Ending Stock = December Ending Stock

YTD Average Stock = (January Starting Stock + Ending Stock per month) / (Amount of Month +1)

How can I do it, within my existing Matrix. Thank you for your help.